Safety systems for nuclear reactors



Feb. 28, 1961 A. E. T. NYE 2,973,458

SAFETY SYSTEMS FOR NUCLEAR REAcToRS Filed Nov. s, 1957 l A7 |557 'SC O Q V$ l L :n f5

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SAFETY sYsrnMs non NUCLEAR nnacrons Anthony Edward Thomas Nye, Clifton, near Rugby, England, assignor to The British Thomson-Houston Corn-N pany Limited, London, England, a British company Filed Nov. 8, 1957, Ser. No. 695,433

Claims priority, application Great Britain Nov. 9, 1956 4 Claims. (Cl. SlT-31) This vinvention relates to safetyy systems for nuclear reactors. When certain faults occur in a nuclear reactor it is necessary to effect an emergency shut-down. The shutting down action may be initiated by a relay which is normally energised from a reliable source through one or more contacts which are closed during normal operation. On theoccurrence f a critical fault however, one or morerof the lcontacts is opened to de-energise the relay, which thereupon initiates shut-down, substantially instantaneous operation` of the relay on de-energisation thereof being required in order that the shut-down will be effected in a minimum of time.

The contacts referred to are commonly provided on control relays connected in a circuit which is energised from anvA.C. source, and since faults developing in the supply system do not justify shutting down the reactor, a duplicate A.C. source may beprovided as a standby which is automatically substituted for the other source in the event, say, of the voltage or frequency departing from a nominal value by more than a predetermined amount. The change-over from one source to the other necessarily occupies a period of time which usually will be longer than the permissible release time of the control relays, this release time being determined by the usually very small safe period for a critical fault. Thus in the event of a supply fault the control relays would become de-energised and bring about shutting down of the reactor. As previously stated this is undesirable and the purpose of the present invention is to provide a safety system which discriminates between critical reactor faults and a fault in the supply system so as not to shut down on the occurrence of such supply fault.

In the following disclosure of the invention reference to a relay as being normally energised relates to the condition of the relay during norrnal'operation of the reactor, whereas reference to relay contacts as being normally open or closed relates to the condition of the contacts in the de-energised condition of their relay, whether or not this is the condition of the relay for normal operation of the reactor.

According tothe present invention a safety system for a nuclear reactor comprises what will'for convenience be termed a shut-down relay having an energising winding arranged to be normally energised from a first source of supply and on de-energisation to initiate shut-down of the reactor, at least one control relay arranged to bc normally energised from one of two interchangeable further sources of supply and to be de-energised on the occurrence of abnormal operating conditions necessitating shut-down,y said cont-rol relay having nor `tally open contacts connected in series with the energising winding of the shut-down relay, and a pair of normally energised relays arranged for de-energisation inv consequence of abnormal conditions in the energising supply for the control relay andv respectively havingY normally open and normally closed contacts connected in series in-a circuit which shunts the control relay contacts connected in series withsaid'shuodown Vrelay winding, tbatone of said ,973,458 Patented Feb. 28, 19611A pair of relays` Whose said contacts are normally openhaving a delayed release time at least as great as the time taken to change over said sources supplying the` control relay whereby during such changeover to main; tain energisation of the shut-down relay throughA said shunt circuit irrespective of de-energisation of the conf trol relay.

in order that the invention may be more fully under stood, a particular embodiment thereof will now be described in conjunction with the accompanying drawing the single ligure of which shows a circuit arrangement` incorporating the invention.

Referring to the drawing, the reference CC designates an energising` and control circuit for a shut-down relay SH of a nuclearreactor, which relay inrknown manner (not shown) controls the supply to the control rods of the reactor and is effective on its de-euergisation to initiate a shut-down action. The circuit CC comprisesV in series with an energising winding sh and normally open contacts Sl-ll` of thcrelay SH, normally open contacts SAL SBl, SCI, and so on (the possible presence of other such contacts being indicated at l) of a number of control relays SA, SB, SC which are responsivev to the occurrence of critical faults in respective sections FA, 13B, FC of the reactor. These control relays have their energising windings sa, sb, sc connected in series with respective reactor section guard circuits (indicated atffa, fb, fc v. across an alternating current supply ac for energisation thereby during normal operationro the reactor, so that their contacts SAI, SBll, SCI establish an energisingcircuit for the relay SH across a reliable direct current source 4de. On occurrence of a fault in the reactor, the' guard circuit in the relevant sectie-n is broken to de-.energise,the'winding of the corresponding control relay, which thereupon releasesv to open its contact inthe control circuit CCand thereby de-energisek the rel-ay SH 'which on release initiates a shutting'down action-for the reactor. Since this action has to. bey effectedas quickly as possible the control re# lays are sofdesigned'f that. their release lag is small. The contact SHE is includedin the energising circuit for its relay to prevent. refoperation thereof irrespective of subsequent closure of the opened control relay contacts. To re-establish the energising circuit for the relay SH, a temporary shunt connection 2 is established in any suit ableL manner across contact SHl until such time as this contact closes on operation of its relay. Connected in a shunt circuit across the control relay contacts SAIL, SB, SCl is a normally open relay contact UVT2 in series with a normally closed relay contact UVl: the purpose of these contacts, which are on 4a supply fault relay UV and a delayed release relay UVT respectively, will be dealt with presently. when the operation offthese relays is being considered.

As aforesaid, the, control relays SA, SB, SC are connected for energisation by the supply ac.' This vsupply is fedfrom a power distribution boardDB which is supplied from one of two alternating current supply sources AC1 and ACZ, constituted for instance by re spective motor driven alternators (not shown), one of which isselected as the normal supply source andthe other vas a standby source; the output from each source is made available at all times, that is, in the case of alternators they are both continuously running. On occurren'ce of abnormal conditions in the supply ac for the control relays, for instance, when an under-voltage o'r over-voltage or a departure from a nominal'frequency occurs, a change-over circuit efects connection of the standby supply source to the power distribution board DB in substitution for the other 'supplysource This change-overcircuit comprises in respect of each supply. source AC1 and ACZ-a-series-crcit (C1 or C2)`1 connected across two phases of the appertaining source (assuming the supply sources to be three-phase as shown). For the supply source AC1 the series circuit C1 follows the path: phase line r, full-wave bridge rectier arrangement Rfa feeding the energising coil coa of a relay COA which when operated connects the supply source AC1 through its normally open contacts COAZ, COA3, and COA4 to the power distribution board DB; normally closed contact COB1 of a corresponding relay COB included in the series circuit for the supply source ACZ; normally closed contact COH1 of a delayed release change-over relay COH; normally open contact UVS ofthe supply fault relay UV; and phase line y of the supply source AC1. Connected in parallel with the contact UVS is a normally open contact COHZ of the relay COH. For the supply source AC2 a similar series circuit C2 is provided over the path: phase line r'; full-wave bridge A rectifier arrangement Rfb feeding the energising coil cob of the relay COB, which relay Vwhen operated connects the supply source AC2 through its normally open contacts COBZ, COB3 and COB4 to the power distribution board DB; normally closed contact COA1 of the relay COA; normally closed contact COH3 of the relay COH, normally open contact UV4 of the relay UV, and phase line y'. Connected in parallel with 'the contact UV4 is a normally open contact COH4 of the relay COH.

The supply fault relay UV has its energising coil uv connected across the supply ac in series with a normally open contact UV3 of the relay UV, norm-ally open contact FRI of a frequency-sensitive relay FR, and normally open contact HV1 of a high voltage relay HV. The relays FR and HV, which have their respective energising windings fr and hv connected directly across the supply ac for operation thereby during normal supply conditions, are respectively `arranged to release, when the supply ac departs from a nominal frequency and when it exceeds a predetermined voltage, thereby breaking at their respective contacts FRl and HV1 the energising circuit for the relay UV; this latter relay itself is normally releasable on the supply ac falling below a predetermined value.

For selectin-g one of the supply sources AC1 and ACZ to supply the power distribution board DB, there is provided a two-position selectionswitch S having contacts S1 associated with the series circuit C1 for the supply -source AC1, and contact S2 associated with the series circuit C2 for the supply source ACZ. With the contacts of the switch S in position 1 the circuit is so arranged that when the supply sources AC1 and ACZ are initially switched on (it being appreciated that there ris no supply ac at this time and that therefore none of the relays con nected across this supply are energised), the relay COB is energised from the supply source AC2 over the series circuit C2, open contact UV4 being shunted by the switch contact S2. Consequently the supply ACZ is connected through contacts COB2, COB3, and COB4 of relay COB to the power distribution board DB. If the contacts of the switch S had been in position 2 the relay COA would have been energised from the supply source AC1 over the series circuit C1 (contactUVS being shunted by Virtue of the position Iof switch contact S1 in thisy instance) to connect this supply through contacts COAZ, COA3, and COA4 to the power distribution board DB.

YFollowing connection'of one otY the supplies AC14 or ACZ to the distribution board DB in the manner just described, the change-over relay COH is operated as by depressing a push button H to connect its energising winding coh across a source V. Assume rstly that the supply sourc AC1 and ACZ have been switched on with the selection switch Sin position l so that relay COB has been operated and at its contacts COBZ, COB3, and COB4 has connected the supply source AC2 to the power distribution board DB. Under these conditions, the supply ac is available at the power distribution board DB and therefore the relay circuits connected across this supply are energised. Relay UV is therefore operated at .this time and contacts UV4 and UVS are closed, contact UV4 providing a hold circuit for relay COB independently of contact S2 of the switch S, while contact UVS prepares an energising circuit for relay lCOA, this latter circuit being incomplete however because contact COB1 is open. When change-over relay COH is operated with the' circuit in this condition, contact COH3 opens to break the energising circuit for relay COB and contact COH2 completes the energising circuit for relay COA once relay COB is released and its contact COB1 is restored to its normally closed position: contacts COH1 and COH4 are inelective `for this switch position. The inherent delay in the changeover operation, even although the output from the supply souce AC1 is immediately available` throughcontacts COAZ, COA3, and COA4 to the power-.distribution board DB, causes a momentary interruption in the supply ac with the result that relay UV will release temporarily during the change-over operation. However, whilerelayfCOH is operated, the energising circuit for relay COA is independent of contact UVS since the latter is shunted by contact COH2. The release time of relay COH is therefore made long enough to bridge the periodV while UV is released. Once the relay UV has re-operated lthe changeover relay COH can be released. The energising circuit for relay COA is then dependent on relay UV remaining operated, and the energising circuit for relay COB is held open at COA1. In this way the source AC1 is established as the normal source providing the supply ac, while the source AC2 constitutes the standby source. Y Y

lf the selection switch S had been in position 2, so that the source'ACl was initially connected to the distribution board DB, operation of the relay COH would have changed over the sources AC1 and ACZ in a similar manner to that just described: that is, contact COH1 would have broken the initially established energising circuit for relay COA, while contact COH4 established an energising circuit for-relay COB. Once the i change-over relay COH 4had been released, the relay COB would remain energised over -contacts COA1, COH3, and UV4, with the energising circuit for relay COA held open at contact COB1. Thus the source ACZ would have been established as the normal source and AC1 as standby.

Y relay UV being operated. Consequently the de-energised condition of relay UVvexisting prior to the establishment of the supply ac, prevents operation of Whichever of the relays COA or COB pertains to the selected normal source Anl or ACZ and thus prevents the supply ac from initially being obtained from that source. It is for this reason that the standby source is initially and temporarily connected to the distribution board DB. Moreover, the arrangement provides a convenient test that the standby source is initially without fault, since if such fault existed the relay UV would fail to operate.

If, after the establishment of oney of the sources AC1 or ACZas the normal source', some abnormal condition arises in the supply ac, the fault relay yUV will release to interrupt the energising circuit for the then energised Vrelay COA or COB at contact UVS or UV4 respectively. Consequent upon this, contact COA1 or COB1 of the released relay COA or COB, completes the energising circuit for the other of these relays so that the standby vsupply source is connected through the contacts thereof to the distribution board DB. It will be seen that in 'either case the energising circuit just mentioned follows a path which shunts the appertaining (UV4 or UV5) contact of the fault relay UV by way of the appertaining contact (S2 or Sl) ofthe selection switch S.

The interruption caused iu the supply ac on a changeover of the supply sources results in cie-energisation of the control relays SA, SB, SC which, in kView of their necessarily short release time as previously explained, break the energising circuit established through their contacts SAI, SE1, SCF` for the shut-down relay SH. To prevent the reactor being shut down unnecessarily in these circumstances, an alternative energising circuit for the shut-down relay SH is temporarily established during the change-over period. To this end, the contacts SAL SBl, SC?. of the control relays are shunted by the circuit consisting of-thc normally closed contact UV of the fault relay UV in series with the normally open contact UVT2 of the delayed release relay UVT. The energising winding uvz of relay UVT is fed from a bridge rectier arrangement Rfc which is connected across the supply ac in series with a normally open contact UVZ of the fault relay UV, the relay UVT thus being de-energised, although not immediately released (by virtue of its release lag), on release of the fault relay UV. In the energising circuit for the relay UV a normally closed contact UVT1 of the delayed release relay UVT, is connected in parallel with the normally open contact UV3 to provide a circuit for initially energising the fault relay UV independently of this latter contact. Following energisation of relay UV its contact UV3 shunts the contact UVT1 so that the energisation is not interrupted by the subsequent opening of contact UVT1 on operation of relay UVT. Should fault relay UV be released, for any of the reasons previously mentioned, it cannot be re-operated until its energising circuit is re-established at contact UVT1 after the delayed release relay UVT has timed out.

Under normal operating conditions the shunt circuit across the control relay contacts SAI, SBl, SCI is open at contact UV1 of the then energised fault relay UV, the delayed release relay contact UVT2 in series therewith being closed. On occurrence of an abnormal condition in the supply ac both these relays become deenergised and the relay UV, in addition to changing over the supply source in the manner previously described, closes its contact UV1 vin the shunt circuit across the control relay contacts SAI, SBI, SC1 Since the contact UVTZ of the relay UVT does not immediately open an alternative energising circuit is temporarily established for the shut-down relay SH through contacts UV1 and UVTZ, preventing its de-energisation even although the control relays SA, SB, SC may release as a result of the interruption of the supply ac. This condition continues until .the relay UVT has timed out by which time the change-over of the supply sources should have been successfully completed. On releasing, relay UVT closes its contact UVT1 to permit re-energisation of relay UV and opens its contact UVT2 to render the control relays SA, SB, SC again effective to control the energisation of the sut-down relay SH. Energisation of relay UV if the standby source is in order then closes contacts UVZ to re-energise relay UVT.

In providing a circuit arrangement incorporating the invention some of the relays and their contacts may be duplicated to ensure safety in case of failure thereof. Moreover spare circuits identical with those described may be provided so that maintenance thereon may be effected without having to shut down the reactor.

What I claim is:

l. A safety system for a nuclear reactor or the like comprising: a shut-down relay having an energizing winding arranged to be normally energized from a rst source of supply and on de-energization to initiate shutdown of the reactor; at least one control relay arranged to be normally energized from one of two interchangeable further sources of supply and to be de-energized on the occurrence of abnormal operating conditions necessitating shut-down, said control relay having a norrnally open contact connected in series with said energizing winding of said shut-down relay; and a pair of normally energized relays arranged for de-energization in consequence of abnormal conditions in the energizing supply for the control relay and respectively having a normally open contact and a normally closed contact connected in series in a circuit which shunts said control relay contact connected in series with said shutdown relay winding, that one of said pair of relays whose said contact is normally open having a delayed release time at least as great as the time taken to change over said sources supplying the control relay whereby during such change-over to maintain energization of the shut-down relay through said shunt circuit irrespective of de-energization of the control relay.

2. A safety system as claimed in claim l wherein said pair of relays comprises a fault relay connected to be normally energized from the same supply as said control relay and to be de-energized in consequence of abnormal conditions therein, and a delayed release relay connected for energization from this same supply through a normally open contact of the fault relay, the fault relay having a normally closed contact in said shunt circuit and Vthe delayed release relay khaving a normally open contact therein.

3. A safety system as claimed in claim 2 wherein said fault relay possesses a normally open contact connected in its own energizing circuit and the delayed release relay has a normally closed contact shunting said normally open contact of the fault relay.

4. A safety system as claimed in claim l, wherein there is provided for said interchangeable ysources of supply a change-over circuit comprising in respect of each of these sources a series connection energizable from the appertaining source and including: the energizing winding of a connection relay effective when energized to establish connection of that source as the control relay supply; a normally closed contact of the connection relay in the corresponding series connection pertaining to the other source; a normally closed contact of change-over switching means; and a parallel connection comprising a normally open contact of said switching means and a normally open contact of one of said relays de-energizable by abnormal control relay supply conditions, which series connections have connected therewith selection switching means having a rst position in which the normally closed change-over contact in one of the series connections and the contacts in said parallel connection in the other series connection are renderedinetectual, and a second position in which the corresponding contacts in the alternate series connections are rendered ineiectual.

References Cited in the tile of this patent UNITED STATES PATENTS 2,683,818 Smith July 13, 1954 

